Rapid Supraglacial Lake Drainages Dewater Isolated Cavities and Decrease Ice Velocity in Western Greenland

Abstract:

While mid-melt season ice deceleration of the Greenland Ice Sheet (GrIS) has been widely attributed to increased channelization of the subglacial drainage system, recent studies indicate the drainage of isolated cavities may instead be responsible. Large meltwater inputs have been documented to dewater isolated cavities on alpine glaciers, but their impact on sliding is likely small compared to the dominant control of channelization on sliding speeds. However, in the GrIS ablation area, low moulin density, shallow surface slopes, and thick ice limit channelization, potentially allowing isolated cavities to exert a stronger control on sliding than on alpine glaciers. We measure moulin hydraulic head, ice motion, and glaciohydraulic tremor at a site over eight kilometers downglacier from rapidly draining supraglacial lakes in the ablation area of Sermeq Avannarleq in west Greenland. Our observations show floodwaters from rapid lake drainages can induce the spatially extensive and persistent alteration of the subglacial drainage system. After subglacial floodwaters drained past our site ice velocity slowed to winter-time background speeds while moulin water levels were still high, indicating pressures within the active drainage system were not responsible for the slowdown. Also, glaciohydraulic tremor amplitude was similar to before the lake drainage, implying the size of the subglacial channel was mostly unchanged, indicating the floodwaters did not increase channelization at our site. Instead, the altered relationship between daily minimum moulin water level and ice velocity suggests the dewatering of isolated cavities was responsible for slowing sliding speeds. These findings further demonstrate that changes within the isolated drainage system rather than increased channelization of the active drainage system is responsible for mitigating the impact of increased melting on GrIS ice dynamics.